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>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-VARIABLES"
>33.4. Using Host Variables</A
></H1
><P
>   In <A
HREF="ecpg-commands.html"
>Section 33.3</A
> you saw how you can execute SQL
   statements from an embedded SQL program.  Some of those statements
   only used fixed values and did not provide a way to insert
   user-supplied values into statements or have the program process
   the values returned by the query.  Those kinds of statements are
   not really useful in real applications.  This section explains in
   detail how you can pass data between your C program and the
   embedded SQL statements using a simple mechanism called
   <I
CLASS="FIRSTTERM"
>host variables</I
>. In an embedded SQL program  we
   consider the SQL statements to be <I
CLASS="FIRSTTERM"
>guests</I
> in the C
   program code which is the <I
CLASS="FIRSTTERM"
>host language</I
>. Therefore
   the variables of the C program are called <I
CLASS="FIRSTTERM"
>host
   variables</I
>.
  </P
><P
>   Another way to exchange values between PostgreSQL backends and ECPG
   applications is the use of SQL descriptors, described
   in <A
HREF="ecpg-descriptors.html"
>Section 33.7</A
>.
  </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-VARIABLES-OVERVIEW"
>33.4.1. Overview</A
></H2
><P
>    Passing data between the C program and the SQL statements is
    particularly simple in embedded SQL.  Instead of having the
    program paste the data into the statement, which entails various
    complications, such as properly quoting the value, you can simply
    write the name of a C variable into the SQL statement, prefixed by
    a colon.  For example:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2);</PRE
><P>
    This statements refers to two C variables named
    <TT
CLASS="VARNAME"
>v1</TT
> and <TT
CLASS="VARNAME"
>v2</TT
> and also uses a
    regular SQL string literal, to illustrate that you are not
    restricted to use one kind of data or the other.
   </P
><P
>    This style of inserting C variables in SQL statements works
    anywhere a value expression is expected in an SQL statement.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-DECLARE-SECTIONS"
>33.4.2. Declare Sections</A
></H2
><P
>    To pass data from the program to the database, for example as
    parameters in a query, or to pass data from the database back to
    the program, the C variables that are intended to contain this
    data need to be declared in specially marked sections, so the
    embedded SQL preprocessor is made aware of them.
   </P
><P
>    This section starts with:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;</PRE
><P>
    and ends with:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL END DECLARE SECTION;</PRE
><P>
    Between those lines, there must be normal C variable declarations,
    such as:
</P><PRE
CLASS="PROGRAMLISTING"
>int   x = 4;
char  foo[16], bar[16];</PRE
><P>
    As you can see, you can optionally assign an initial value to the variable.
    The variable's scope is determined by the location of its declaring
    section within the program.
    You can also declare variables with the following syntax which implicitly
    creates a declare section:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL int i = 4;</PRE
><P>
    You can have as many declare sections in a program as you like.
   </P
><P
>    The declarations are also echoed to the output file as normal C
    variables, so there's no need to declare them again.  Variables
    that are not intended to be used in SQL commands can be declared
    normally outside these special sections.
   </P
><P
>    The definition of a structure or union also must be listed inside
    a <TT
CLASS="LITERAL"
>DECLARE</TT
> section. Otherwise the preprocessor cannot
    handle these types since it does not know the definition.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-RETRIEVING"
>33.4.3. Retrieving Query Results</A
></H2
><P
>    Now you should be able to pass data generated by your program into
    an SQL command.  But how do you retrieve the results of a query?
    For that purpose, embedded SQL provides special variants of the
    usual commands <TT
CLASS="COMMAND"
>SELECT</TT
> and
    <TT
CLASS="COMMAND"
>FETCH</TT
>.  These commands have a special
    <TT
CLASS="LITERAL"
>INTO</TT
> clause that specifies which host variables
    the retrieved values are to be stored in.
    <TT
CLASS="COMMAND"
>SELECT</TT
> is used for a query that returns only
    single row, and <TT
CLASS="COMMAND"
>FETCH</TT
> is used for a query that
    returns multiple rows, using a cursor.
   </P
><P
>    Here is an example:
</P><PRE
CLASS="PROGRAMLISTING"
>/*
 * assume this table:
 * CREATE TABLE test1 (a int, b varchar(50));
 */

EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;

 ...

EXEC SQL SELECT a, b INTO :v1, :v2 FROM test;</PRE
><P>
    So the <TT
CLASS="LITERAL"
>INTO</TT
> clause appears between the select
    list and the <TT
CLASS="LITERAL"
>FROM</TT
> clause.  The number of
    elements in the select list and the list after
    <TT
CLASS="LITERAL"
>INTO</TT
> (also called the target list) must be
    equal.
   </P
><P
>    Here is an example using the command <TT
CLASS="COMMAND"
>FETCH</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;

 ...

EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test;

 ...

do
{
    ...
    EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2;
    ...
} while (...);</PRE
><P>
    Here the <TT
CLASS="LITERAL"
>INTO</TT
> clause appears after all the
    normal clauses.
   </P
><P
>    Both of these methods only allow retrieving one row at a time.  If
    you need to process result sets that potentially contain more than
    one row, you need to use a cursor, as shown in the second example.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-VARIABLES-TYPE-MAPPING"
>33.4.4. Type Mapping</A
></H2
><P
>    When ECPG applications exchange values between the PostgreSQL
    server and the C application, such as when retrieving query
    results from the server or executing SQL statements with input
    parameters, the values need to be converted between PostgreSQL
    data types and host language variable types (C language data
    types, concretely).  One of the main points of ECPG is that it
    takes care of this automatically in most cases.
   </P
><P
>    In this respect, there are two kinds of data types: Some simple
    PostgreSQL data types, such as <TT
CLASS="TYPE"
>integer</TT
>
    and <TT
CLASS="TYPE"
>text</TT
>, can be read and written by the application
    directly.  Other PostgreSQL data types, such
    as <TT
CLASS="TYPE"
>timestamp</TT
> and <TT
CLASS="TYPE"
>numeric</TT
> can only be
    accessed through special library functions; see
    <A
HREF="ecpg-variables.html#ECPG-SPECIAL-TYPES"
>Section 33.4.4.2</A
>.
   </P
><P
>    <A
HREF="ecpg-variables.html#ECPG-DATATYPE-HOSTVARS-TABLE"
>Table 33-1</A
> shows which PostgreSQL
    data types correspond to which C data types.  When you wish to
    send or receive a value of a given PostgreSQL data type, you
    should declare a C variable of the corresponding C data type in
    the declare section.
   </P
><DIV
CLASS="TABLE"
><A
NAME="ECPG-DATATYPE-HOSTVARS-TABLE"
></A
><P
><B
>Table 33-1. Mapping Between PostgreSQL Data Types and C Variable Types</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>PostgreSQL data type</TH
><TH
>Host variable type</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="TYPE"
>smallint</TT
></TD
><TD
><TT
CLASS="TYPE"
>short</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>integer</TT
></TD
><TD
><TT
CLASS="TYPE"
>int</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
><TT
CLASS="TYPE"
>long long int</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>decimal</TT
></TD
><TD
><TT
CLASS="TYPE"
>decimal</TT
><A
NAME="ECPG-DATATYPE-TABLE-FN"
HREF="#FTN.ECPG-DATATYPE-TABLE-FN"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>numeric</TT
></TD
><TD
><TT
CLASS="TYPE"
>numeric</TT
><A
HREF="ecpg-variables.html#FTN.ECPG-DATATYPE-TABLE-FN"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>real</TT
></TD
><TD
><TT
CLASS="TYPE"
>float</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>double precision</TT
></TD
><TD
><TT
CLASS="TYPE"
>double</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>serial</TT
></TD
><TD
><TT
CLASS="TYPE"
>int</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>bigserial</TT
></TD
><TD
><TT
CLASS="TYPE"
>long long int</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="TYPE"
>unsigned int</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>character(<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)</TT
>, <TT
CLASS="TYPE"
>varchar(<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)</TT
>, <TT
CLASS="TYPE"
>text</TT
></TD
><TD
><TT
CLASS="TYPE"
>char[<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>+1]</TT
>, <TT
CLASS="TYPE"
>VARCHAR[<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>+1]</TT
><A
NAME="AEN40509"
HREF="#FTN.AEN40509"
><SPAN
CLASS="footnote"
>[b]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
><TT
CLASS="TYPE"
>char[NAMEDATALEN]</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>timestamp</TT
></TD
><TD
><TT
CLASS="TYPE"
>timestamp</TT
><A
HREF="ecpg-variables.html#FTN.ECPG-DATATYPE-TABLE-FN"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>interval</TT
></TD
><TD
><TT
CLASS="TYPE"
>interval</TT
><A
HREF="ecpg-variables.html#FTN.ECPG-DATATYPE-TABLE-FN"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>date</TT
></TD
><TD
><TT
CLASS="TYPE"
>date</TT
><A
HREF="ecpg-variables.html#FTN.ECPG-DATATYPE-TABLE-FN"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
><A
NAME="AEN40540"
HREF="#FTN.AEN40540"
><SPAN
CLASS="footnote"
>[c]</SPAN
></A
></TD
></TR
></TBODY
><TR
><TD
COLSPAN="2"
>Notes:<BR><A
NAME="FTN.ECPG-DATATYPE-TABLE-FN"
>a. </A
>This type can only be accessed through special library functions; see <A
HREF="ecpg-variables.html#ECPG-SPECIAL-TYPES"
>Section 33.4.4.2</A
>.<BR><A
NAME="FTN.AEN40509"
>b. </A
>declared in <TT
CLASS="FILENAME"
>ecpglib.h</TT
><BR><A
NAME="FTN.AEN40540"
>c. </A
>declared in <TT
CLASS="FILENAME"
>ecpglib.h</TT
> if not native<BR></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-CHAR"
>33.4.4.1. Handling Character Strings</A
></H3
><P
>     To handle SQL character string data types, such
     as <TT
CLASS="TYPE"
>varchar</TT
> and <TT
CLASS="TYPE"
>text</TT
>, there are two
     possible ways to declare the host variables.
    </P
><P
>     One way is using <TT
CLASS="TYPE"
>char[]</TT
>, an array
     of <TT
CLASS="TYPE"
>char</TT
>, which is the most common way to handle
     character data in C.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    char str[50];
EXEC SQL END DECLARE SECTION;</PRE
><P>
     Note that you have to take care of the length yourself.  If you
     use this host variable as the target variable of a query which
     returns a string with more than 49 characters, a buffer overflow
     occurs.
    </P
><P
>     The other way is using the <TT
CLASS="TYPE"
>VARCHAR</TT
> type, which is a
     special type provided by ECPG.  The definition on an array of
     type <TT
CLASS="TYPE"
>VARCHAR</TT
> is converted into a
     named <TT
CLASS="TYPE"
>struct</TT
> for every variable. A declaration like:
</P><PRE
CLASS="PROGRAMLISTING"
>VARCHAR var[180];</PRE
><P>
     is converted into:
</P><PRE
CLASS="PROGRAMLISTING"
>struct varchar_var { int len; char arr[180]; } var;</PRE
><P>
     The member <TT
CLASS="STRUCTFIELD"
>arr</TT
> hosts the string
     including a terminating zero byte.  Thus, to store a string in
     a <TT
CLASS="TYPE"
>VARCHAR</TT
> host variable, the host variable has to be
     declared with the length including the zero byte terminator.  The
     member <TT
CLASS="STRUCTFIELD"
>len</TT
> holds the length of the
     string stored in the <TT
CLASS="STRUCTFIELD"
>arr</TT
> without the
     terminating zero byte.  When a host variable is used as input for
     a query, if <TT
CLASS="LITERAL"
>strlen(arr)</TT
>
     and <TT
CLASS="STRUCTFIELD"
>len</TT
> are different, the shorter one
     is used.
    </P
><P
>     Two or more <TT
CLASS="TYPE"
>VARCHAR</TT
> host variables cannot be defined
     in single line statement.  The following code will confuse
     the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor:
</P><PRE
CLASS="PROGRAMLISTING"
>VARCHAR v1[128], v2[128];   /* WRONG */</PRE
><P>
     Two variables should be defined in separate statements like this:
</P><PRE
CLASS="PROGRAMLISTING"
>VARCHAR v1[128];
VARCHAR v2[128];</PRE
><P>
    </P
><P
>     <TT
CLASS="TYPE"
>VARCHAR</TT
> can be written in upper or lower case, but
     not in mixed case.
    </P
><P
>     <TT
CLASS="TYPE"
>char</TT
> and <TT
CLASS="TYPE"
>VARCHAR</TT
> host variables can
     also hold values of other SQL types, which will be stored in
     their string forms.
    </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-SPECIAL-TYPES"
>33.4.4.2. Accessing Special Data Types</A
></H3
><P
>     ECPG contains some special types that help you to interact easily
     with some special data types from the PostgreSQL server. In
     particular, it has implemented support for the
     <TT
CLASS="TYPE"
>numeric</TT
>, <TT
CLASS="TYPE"
>decimal</TT
>, <TT
CLASS="TYPE"
>date</TT
>, <TT
CLASS="TYPE"
>timestamp</TT
>,
     and <TT
CLASS="TYPE"
>interval</TT
> types.  These data types cannot usefully be
     mapped to primitive host variable types (such
     as <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="TYPE"
>long long int</TT
>,
     or <TT
CLASS="TYPE"
>char[]</TT
>), because they have a complex internal
     structure.  Applications deal with these types by declaring host
     variables in special types and accessing them using functions in
     the pgtypes library.  The pgtypes library, described in detail
     in <A
HREF="ecpg-pgtypes.html"
>Section 33.6</A
> contains basic functions to deal
     with those types, such that you do not need to send a query to
     the SQL server just for adding an interval to a time stamp for
     example.
    </P
><P
>     The follow subsections describe these special data types. For
     more details about pgtypes library functions,
     see <A
HREF="ecpg-pgtypes.html"
>Section 33.6</A
>.
    </P
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="AEN40588"
>33.4.4.2.1. timestamp, date</A
></H4
><P
>      Here is a pattern for handling <TT
CLASS="TYPE"
>timestamp</TT
> variables
      in the ECPG host application.
     </P
><P
>      First, the program has to include the header file for the
      <TT
CLASS="TYPE"
>timestamp</TT
> type:
</P><PRE
CLASS="PROGRAMLISTING"
>#include &lt;pgtypes_timestamp.h&#62;</PRE
><P>
     </P
><P
>      Next, declare a host variable as type <TT
CLASS="TYPE"
>timestamp</TT
> in
      the declare section:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
timestamp ts;
EXEC SQL END DECLARE SECTION;</PRE
><P>
     </P
><P
>      And after reading a value into the host variable, process it
      using pgtypes library functions. In following example, the
      <TT
CLASS="TYPE"
>timestamp</TT
> value is converted into text (ASCII) form
      with the <CODE
CLASS="FUNCTION"
>PGTYPEStimestamp_to_asc()</CODE
>
      function:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL SELECT now()::timestamp INTO :ts;

printf("ts = %s\n", PGTYPEStimestamp_to_asc(ts));</PRE
><P>
      This example will show some result like following:
</P><PRE
CLASS="SCREEN"
>ts = 2010-06-27 18:03:56.949343</PRE
><P>
     </P
><P
>      In addition, the DATE type can be handled in the same way. The
      program has to include <TT
CLASS="FILENAME"
>pg_types_date.h</TT
>, declare a host variable
      as the date type and convert a DATE value into a text form using
      <CODE
CLASS="FUNCTION"
>PGTYPESdate_to_asc()</CODE
> function. For more details about the
      pgtypes library functions, see <A
HREF="ecpg-pgtypes.html"
>Section 33.6</A
>.
     </P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-TYPE-INTERVAL"
>33.4.4.2.2. interval</A
></H4
><P
>      The handling of the <TT
CLASS="TYPE"
>interval</TT
> type is also similar
      to the <TT
CLASS="TYPE"
>timestamp</TT
> and <TT
CLASS="TYPE"
>date</TT
> types.  It
      is required, however, to allocate memory for
      an <TT
CLASS="TYPE"
>interval</TT
> type value explicitly.  In other words,
      the memory space for the variable has to be allocated in the
      heap memory, not in the stack memory.
     </P
><P
>      Here is an example program:
</P><PRE
CLASS="PROGRAMLISTING"
>#include &lt;stdio.h&#62;
#include &lt;stdlib.h&#62;
#include &lt;pgtypes_interval.h&#62;

int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
    interval *in;
EXEC SQL END DECLARE SECTION;

    EXEC SQL CONNECT TO testdb;

    in = PGTYPESinterval_new();
    EXEC SQL SELECT '1 min'::interval INTO :in;
    printf("interval = %s\n", PGTYPESinterval_to_asc(in));
    PGTYPESinterval_free(in);

    EXEC SQL COMMIT;
    EXEC SQL DISCONNECT ALL;
    return 0;
}</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-TYPE-NUMERIC-DECIMAL"
>33.4.4.2.3. numeric, decimal</A
></H4
><P
>      The handling of the <TT
CLASS="TYPE"
>numeric</TT
>
      and <TT
CLASS="TYPE"
>decimal</TT
> types is similar to the
      <TT
CLASS="TYPE"
>interval</TT
> type: It requires defining a pointer,
      allocating some memory space on the heap, and accessing the
      variable using the pgtypes library functions.  For more details
      about the pgtypes library functions,
      see <A
HREF="ecpg-pgtypes.html"
>Section 33.6</A
>.
     </P
><P
>      No functions are provided specifically for
      the <TT
CLASS="TYPE"
>decimal</TT
> type.  An application has to convert it
      to a <TT
CLASS="TYPE"
>numeric</TT
> variable using a pgtypes library
      function to do further processing.
     </P
><P
>      Here is an example program handling <TT
CLASS="TYPE"
>numeric</TT
>
      and <TT
CLASS="TYPE"
>decimal</TT
> type variables.
</P><PRE
CLASS="PROGRAMLISTING"
>#include &lt;stdio.h&#62;
#include &lt;stdlib.h&#62;
#include &lt;pgtypes_numeric.h&#62;

EXEC SQL WHENEVER SQLERROR STOP;

int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
    numeric *num;
    numeric *num2;
    decimal *dec;
EXEC SQL END DECLARE SECTION;

    EXEC SQL CONNECT TO testdb;

    num = PGTYPESnumeric_new();
    dec = PGTYPESdecimal_new();

    EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec;

    printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
    printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
    printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));

    /* Convert decimal to numeric to show a decimal value. */
    num2 = PGTYPESnumeric_new();
    PGTYPESnumeric_from_decimal(dec, num2);

    printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
    printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
    printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));

    PGTYPESnumeric_free(num2);
    PGTYPESdecimal_free(dec);
    PGTYPESnumeric_free(num);

    EXEC SQL COMMIT;
    EXEC SQL DISCONNECT ALL;
    return 0;
}</PRE
><P>
     </P
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-VARIABLES-NONPRIMITIVE-C"
>33.4.4.3. Host Variables with Nonprimitive Types</A
></H3
><P
>     As a host variable you can also use arrays, typedefs, structs, and
     pointers.
    </P
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-VARIABLES-ARRAYS"
>33.4.4.3.1. Arrays</A
></H4
><P
>      There are two use cases for arrays as host variables.  The first
      is a way to store some text string in <TT
CLASS="TYPE"
>char[]</TT
>
      or <TT
CLASS="TYPE"
>VARCHAR[]</TT
>, as
      explained <A
HREF="ecpg-variables.html#ECPG-CHAR"
>Section 33.4.4.1</A
>.  The second use case is to
      retrieve multiple rows from a query result without using a
      cursor.  Without an array, to process a query result consisting
      of multiple rows, it is required to use a cursor and
      the <TT
CLASS="COMMAND"
>FETCH</TT
> command.  But with array host
      variables, multiple rows can be received at once.  The length of
      the array has to be defined to be able to accommodate all rows,
      otherwise a buffer overflow will likely occur.
     </P
><P
>      Following example scans the <TT
CLASS="LITERAL"
>pg_database</TT
>
      system table and shows all OIDs and names of the available
      databases:
</P><PRE
CLASS="PROGRAMLISTING"
>int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
    int dbid[8];
    char dbname[8][16];
    int i;
EXEC SQL END DECLARE SECTION;

    memset(dbname, 0, sizeof(char)* 16 * 8);
    memset(dbid, 0, sizeof(int) * 8);

    EXEC SQL CONNECT TO testdb;

    /* Retrieve multiple rows into arrays at once. */
    EXEC SQL SELECT oid,datname INTO :dbid, :dbname FROM pg_database;

    for (i = 0; i &lt; 8; i++)
        printf("oid=%d, dbname=%s\n", dbid[i], dbname[i]);

    EXEC SQL COMMIT;
    EXEC SQL DISCONNECT ALL;
    return 0;
}</PRE
><P>

    This example shows following result. (The exact values depend on
    local circumstances.)
</P><PRE
CLASS="SCREEN"
>oid=1, dbname=template1
oid=11510, dbname=template0
oid=11511, dbname=postgres
oid=313780, dbname=testdb
oid=0, dbname=
oid=0, dbname=
oid=0, dbname=</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-VARIABLES-STRUCT"
>33.4.4.3.2. Structures</A
></H4
><P
>      A structure whose member names match the column names of a query
      result, can be used to retrieve multiple columns at once.  The
      structure enables handling multiple column values in a single
      host variable.
     </P
><P
>      The following example retrieves OIDs, names, and sizes of the
      available databases from the <TT
CLASS="LITERAL"
>pg_database</TT
>
      system table and using
      the <CODE
CLASS="FUNCTION"
>pg_database_size()</CODE
> function.  In this
      example, a structure variable <TT
CLASS="VARNAME"
>dbinfo_t</TT
> with
      members whose names match each column in
      the <TT
CLASS="LITERAL"
>SELECT</TT
> result is used to retrieve one
      result row without putting multiple host variables in
      the <TT
CLASS="LITERAL"
>FETCH</TT
> statement.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    typedef struct
    {
       int oid;
       char datname[65];
       long long int size;
    } dbinfo_t;

    dbinfo_t dbval;
EXEC SQL END DECLARE SECTION;

    memset(&amp;dbval, 0, sizeof(dbinfo_t));

    EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
    EXEC SQL OPEN cur1;

    /* when end of result set reached, break out of while loop */
    EXEC SQL WHENEVER NOT FOUND DO BREAK;

    while (1)
    {
        /* Fetch multiple columns into one structure. */
        EXEC SQL FETCH FROM cur1 INTO :dbval;

        /* Print members of the structure. */
        printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, dbval.size);
    }

    EXEC SQL CLOSE cur1;</PRE
><P>
     </P
><P
>      This example shows following result. (The exact values depend on
      local circumstances.)
</P><PRE
CLASS="SCREEN"
>oid=1, datname=template1, size=4324580
oid=11510, datname=template0, size=4243460
oid=11511, datname=postgres, size=4324580
oid=313780, datname=testdb, size=8183012</PRE
><P>
     </P
><P
>      Structure host variables <SPAN
CLASS="QUOTE"
>"absorb"</SPAN
> as many columns
      as the structure as fields.  Additional columns can be assigned
      to other host variables. For example, the above program could
      also be restructured like this, with the <TT
CLASS="VARNAME"
>size</TT
>
      variable outside the structure:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    typedef struct
    {
       int oid;
       char datname[65];
    } dbinfo_t;

    dbinfo_t dbval;
    long long int size;
EXEC SQL END DECLARE SECTION;

    memset(&amp;dbval, 0, sizeof(dbinfo_t));

    EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
    EXEC SQL OPEN cur1;

    /* when end of result set reached, break out of while loop */
    EXEC SQL WHENEVER NOT FOUND DO BREAK;

    while (1)
    {
        /* Fetch multiple columns into one structure. */
        EXEC SQL FETCH FROM cur1 INTO :dbval, :size;

        /* Print members of the structure. */
        printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, size);
    }

    EXEC SQL CLOSE cur1;</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="AEN40660"
>33.4.4.3.3. Typedefs</A
></H4
><P
>      Use the <TT
CLASS="LITERAL"
>typedef</TT
> keyword to map new types to already
      existing types.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    typedef char mychartype[40];
    typedef long serial_t;
EXEC SQL END DECLARE SECTION;</PRE
><P>
      Note that you could also use:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL TYPE serial_t IS long;</PRE
><P>
      This declaration does not need to be part of a declare section.
     </P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="AEN40666"
>33.4.4.3.4. Pointers</A
></H4
><P
>      You can declare pointers to the most common types. Note however
      that you cannot use pointers as target variables of queries
      without auto-allocation. See <A
HREF="ecpg-descriptors.html"
>Section 33.7</A
>
      for more information on auto-allocation.
     </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    int   *intp;
    char **charp;
EXEC SQL END DECLARE SECTION;</PRE
><P>
     </P
></DIV
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-VARIABLES-NONPRIMITIVE-SQL"
>33.4.5. Handling Nonprimitive SQL Data Types</A
></H2
><P
>    This section contains information on how to handle nonscalar and
    user-defined SQL-level data types in ECPG applications.  Note that
    this is distinct from the handling of host variables of
    nonprimitive types, described in the previous section.
   </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN40675"
>33.4.5.1. Arrays</A
></H3
><P
>     SQL-level arrays are not directly supported in ECPG.  It is not
     possible to simply map an SQL array into a C array host variable.
     This will result in undefined behavior.  Some workarounds exist,
     however.
    </P
><P
>     If a query accesses <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>elements</I
></SPAN
> of an array
     separately, then this avoids the use of arrays in ECPG.  Then, a
     host variable with a type that can be mapped to the element type
     should be used.  For example, if a column type is array of
     <TT
CLASS="TYPE"
>integer</TT
>, a host variable of type <TT
CLASS="TYPE"
>int</TT
>
     can be used.  Also if the element type is <TT
CLASS="TYPE"
>varchar</TT
>
     or <TT
CLASS="TYPE"
>text</TT
>, a host variable of type <TT
CLASS="TYPE"
>char[]</TT
>
     or <TT
CLASS="TYPE"
>VARCHAR[]</TT
> can be used.
    </P
><P
>     Here is an example.  Assume the following table:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE t3 (
    ii integer[]
);

testdb=&#62; SELECT * FROM t3;
     ii
-------------
 {1,2,3,4,5}
(1 row)</PRE
><P>

     The following example program retrieves the 4th element of the
     array and stores it into a host variable of
     type <TT
CLASS="TYPE"
>int</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int ii;
EXEC SQL END DECLARE SECTION;

EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[4] FROM t3;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    EXEC SQL FETCH FROM cur1 INTO :ii ;
    printf("ii=%d\n", ii);
}

EXEC SQL CLOSE cur1;</PRE
><P>

     This example shows the following result:
</P><PRE
CLASS="SCREEN"
>ii=4</PRE
><P>
    </P
><P
>     To map multiple array elements to the multiple elements in an
     array type host variables each element of array column and each
     element of the host variable array have to be managed separately,
     for example:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;

EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[1], ii[2], ii[3], ii[4] FROM t3;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    EXEC SQL FETCH FROM cur1 INTO :ii_a[0], :ii_a[1], :ii_a[2], :ii_a[3];
    ...
}</PRE
><P>
    </P
><P
>     Note again that
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;

EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii FROM t3;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    /* WRONG */
    EXEC SQL FETCH FROM cur1 INTO :ii_a;
    ...
}</PRE
><P>
     would not work correctly in this case, because you cannot map an
     array type column to an array host variable directly.
    </P
><P
>     Another workaround is to store arrays in their external string
     representation in host variables of type <TT
CLASS="TYPE"
>char[]</TT
>
     or <TT
CLASS="TYPE"
>VARCHAR[]</TT
>.  For more details about this
     representation, see <A
HREF="arrays.html#ARRAYS-INPUT"
>Section 8.14.2</A
>.  Note that
     this means that the array cannot be accessed naturally as an
     array in the host program (without further processing that parses
     the text representation).
    </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN40699"
>33.4.5.2. Composite Types</A
></H3
><P
>     Composite types are not directly supported in ECPG, but an easy workaround is possible.
  The
     available workarounds are similar to the ones described for
     arrays above: Either access each attribute separately or use the
     external string representation.
    </P
><P
>     For the following examples, assume the following type and table:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TYPE comp_t AS (intval integer, textval varchar(32));
CREATE TABLE t4 (compval comp_t);
INSERT INTO t4 VALUES ( (256, 'PostgreSQL') );</PRE
><P>

     The most obvious solution is to access each attribute separately.
     The following program retrieves data from the example table by
     selecting each attribute of the type <TT
CLASS="TYPE"
>comp_t</TT
>
     separately:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int intval;
varchar textval[33];
EXEC SQL END DECLARE SECTION;

/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    /* Fetch each element of the composite type column into host variables. */
    EXEC SQL FETCH FROM cur1 INTO :intval, :textval;

    printf("intval=%d, textval=%s\n", intval, textval.arr);
}

EXEC SQL CLOSE cur1;</PRE
><P>
    </P
><P
>     To enhance this example, the host variables to store values in
     the <TT
CLASS="COMMAND"
>FETCH</TT
> command can be gathered into one
     structure.  For more details about the host variable in the
     structure form, see <A
HREF="ecpg-variables.html#ECPG-VARIABLES-STRUCT"
>Section 33.4.4.3.2</A
>.
     To switch to the structure, the example can be modified as below.
     The two host variables, <TT
CLASS="VARNAME"
>intval</TT
>
     and <TT
CLASS="VARNAME"
>textval</TT
>, become members of
     the <TT
CLASS="STRUCTNAME"
>comp_t</TT
> structure, and the structure
     is specified on the <TT
CLASS="COMMAND"
>FETCH</TT
> command.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
    int intval;
    varchar textval[33];
} comp_t;

comp_t compval;
EXEC SQL END DECLARE SECTION;

/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    /* Put all values in the SELECT list into one structure. */
    EXEC SQL FETCH FROM cur1 INTO :compval;

    printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}

EXEC SQL CLOSE cur1;</PRE
><P>

     Although a structure is used in the <TT
CLASS="COMMAND"
>FETCH</TT
>
     command, the attribute names in the <TT
CLASS="COMMAND"
>SELECT</TT
>
     clause are specified one by one.  This can be enhanced by using
     a <TT
CLASS="LITERAL"
>*</TT
> to ask for all attributes of the composite
     type value.
</P><PRE
CLASS="PROGRAMLISTING"
>...
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).* FROM t4;
EXEC SQL OPEN cur1;

EXEC SQL WHENEVER NOT FOUND DO BREAK;

while (1)
{
    /* Put all values in the SELECT list into one structure. */
    EXEC SQL FETCH FROM cur1 INTO :compval;

    printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
...</PRE
><P>
     This way, composite types can be mapped into structures almost
     seamlessly, even though ECPG does not understand the composite
     type itself.
    </P
><P
>     Finally, it is also possible to store composite type values in
     their external string representation in host variables of
     type <TT
CLASS="TYPE"
>char[]</TT
> or <TT
CLASS="TYPE"
>VARCHAR[]</TT
>.  But that
     way, it is not easily possible to access the fields of the value
     from the host program.
    </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN40721"
>33.4.5.3. User-defined Base Types</A
></H3
><P
>     New user-defined base types are not directly supported by ECPG.
     You can use the external string representation and host variables
     of type <TT
CLASS="TYPE"
>char[]</TT
> or <TT
CLASS="TYPE"
>VARCHAR[]</TT
>, and this
     solution is indeed appropriate and sufficient for many types.
    </P
><P
>     Here is an example using the data type <TT
CLASS="TYPE"
>complex</TT
> from
     the example in <A
HREF="xtypes.html"
>Section 35.11</A
>.  The external string
     representation of that type is <TT
CLASS="LITERAL"
>(%lf,%lf)</TT
>,
     which is defined in the
     functions <CODE
CLASS="FUNCTION"
>complex_in()</CODE
>
     and <CODE
CLASS="FUNCTION"
>complex_out()</CODE
> functions
     in <A
HREF="xtypes.html"
>Section 35.11</A
>.  The following example inserts the
     complex type values <TT
CLASS="LITERAL"
>(1,1)</TT
>
     and <TT
CLASS="LITERAL"
>(3,3)</TT
> into the
     columns <TT
CLASS="LITERAL"
>a</TT
> and <TT
CLASS="LITERAL"
>b</TT
>, and select
     them from the table after that.

</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
    varchar a[64];
    varchar b[64];
EXEC SQL END DECLARE SECTION;

    EXEC SQL INSERT INTO test_complex VALUES ('(1,1)', '(3,3)');

    EXEC SQL DECLARE cur1 CURSOR FOR SELECT a, b FROM test_complex;
    EXEC SQL OPEN cur1;

    EXEC SQL WHENEVER NOT FOUND DO BREAK;

    while (1)
    {
        EXEC SQL FETCH FROM cur1 INTO :a, :b;
        printf("a=%s, b=%s\n", a.arr, b.arr);
    }

    EXEC SQL CLOSE cur1;</PRE
><P>

     This example shows following result:
</P><PRE
CLASS="SCREEN"
>a=(1,1), b=(3,3)</PRE
><P>
   </P
><P
>     Another workaround is avoiding the direct use of the user-defined
     types in ECPG and instead create a function or cast that converts
     between the user-defined type and a primitive type that ECPG can
     handle.  Note, however, that type casts, especially implicit
     ones, should be introduced into the type system very carefully.
    </P
><P
>     For example,
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION create_complex(r double, i double) RETURNS complex
LANGUAGE SQL
IMMUTABLE
AS $$ SELECT $1 * complex '(1,0')' + $2 * complex '(0,1)' $$;</PRE
><P>
    After this definition, the following
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
double a, b, c, d;
EXEC SQL END DECLARE SECTION;

a = 1;
b = 2;
c = 3;
d = 4;

EXEC SQL INSERT INTO test_complex VALUES (create_complex(:a, :b), create_complex(:c, :d));</PRE
><P>
    has the same effect as
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL INSERT INTO test_complex VALUES ('(1,2)', '(3,4)');</PRE
><P>
    </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-INDICATORS"
>33.4.6. Indicators</A
></H2
><P
>    The examples above do not handle null values.  In fact, the
    retrieval examples will raise an error if they fetch a null value
    from the database.  To be able to pass null values to the database
    or retrieve null values from the database, you need to append a
    second host variable specification to each host variable that
    contains data.  This second host variable is called the
    <I
CLASS="FIRSTTERM"
>indicator</I
> and contains a flag that tells
    whether the datum is null, in which case the value of the real
    host variable is ignored.  Here is an example that handles the
    retrieval of null values correctly:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
VARCHAR val;
int val_ind;
EXEC SQL END DECLARE SECTION:

 ...

EXEC SQL SELECT b INTO :val :val_ind FROM test1;</PRE
><P>
    The indicator variable <TT
CLASS="VARNAME"
>val_ind</TT
> will be zero if
    the value was not null, and it will be negative if the value was
    null.
   </P
><P
>    The indicator has another function: if the indicator value is
    positive, it means that the value is not null, but it was
    truncated when it was stored in the host variable.
   </P
><P
>    If the argument <TT
CLASS="LITERAL"
>-r no_indicator</TT
> is passed to
    the preprocessor <TT
CLASS="COMMAND"
>ecpg</TT
>, it works in
    <SPAN
CLASS="QUOTE"
>"no-indicator"</SPAN
> mode. In no-indicator mode, if no
    indicator variable is specified, null values are signaled (on
    input and output) for character string types as empty string and
    for integer types as the lowest possible value for type (for
    example, <TT
CLASS="SYMBOL"
>INT_MIN</TT
> for <TT
CLASS="TYPE"
>int</TT
>).
   </P
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